1. Field of the Invention
The present invention relates to zoom lenses and more particularly to a multiple field of view telescope without moving parts.
2. Description of the Related Art
Typically zoom lenses are not used in space applications because they are unreliable for spaceflight or are relatively heavy. Most space applications involve the use of two complete cameras rather than zoom lenses.
The major problem for space applications is that complex rotating mechanisms used in zoom lenses (or old-fashioned movie cameras with multiple lenses) are at high risk in the changing pressure and temperature conditions found in space flight. Such lenses are typically not used in space missions. Glass zoom lenses are also typically heavy, and thus not suitable for space missions.
Most traditional zoom lens designs have three moving groups: a zoom group, or variator, that changes the effective focal length (EFL); a compensating group that maintains a constant back focus as the zoom group moves; and a focus group in front of the zoom group that lets the zoom group see a virtual object at a fixed conjugate regardless of the actual object distance. This arrangement is desirable for manually operated zoom lenses, since a cam can cause the compensating group to track the zoom group, so that zoom and focus are independent functions.
However, it is not desirable for a spaceborne zoom lens. A mechanical cam has reliability problems with long-term operation in a hard vacuum; without a cam, three independent motor drives would be required. Each drive adds weight to the system and requires power. Also, every additional device degrades the mean time between failures (MTBF) of the system.
OCA Applied Optics developed a zoom lens for the Space Station Mobile servicing system video camera that obviated the need for a cam. (See paper entitled xe2x80x9cDesign of a space-qualified zoom lens for the Space Station Mobile Servicing System video cameraxe2x80x9d published in Zoom Lenses, Pages 37-63, Proc. SPIE Volume 2539, 1995, Anthony B. Hull et al.) The zoom lens used the compensating group to refocus as a function of zoom group position as well as object distance. This required only two moving groups and two motors, which gave rise to significantly enhanced reliability. However, this system still requires two linear stages (drives) and elaborate software in order to control the lens elements that move on these linear stages (instead of a cam).
What is desired is a lightweight system that can change focal lengths without moving parts.
In a broad aspect, the optical system comprises a first set of optics, an optical switch and a second set of optics. The first set of optics is capable of receiving light and directing the light along a first optical path. The optical switch is capable of directing a first portion of the light to continue along the first optical path, and directing a second portion of the light along a second optical path. The second set of optics is subsequent to the optical switch and is capable of receiving the first portion of light from the optical switch, and directing that portion of light along a third optical path. The third optical path and the second optical path are substantially co-aligned. The optical system has variable magnifications and requires no moving parts. In some embodiments, a first set of optical components is capable of accepting an optical input and producing an output having a first image height or magnification. A second set of optical components, forming a second half of the system, accepts the output of the first set of optics and produces an output having different image height or magnification. In one state of the system, light travels through both sets of optics to produce an output. In a second state of the system, an optical switch bypasses the second set. Thus, the same system is capable of producing two different outputs.
In some embodiments, an optical switch, capable of reflecting or transmitting light is placed between the primary mirror and secondary mirror of a reflecting telescope. The optical switch can be used to bring a converging beam from the primary mirror to a focus, thus producing an image having one height, or the switch can be used to allow light to travel to the secondary mirror and converge to an image, having a different height. In some embodiments, the primary and secondary mirror can be arranged so that the two images occur at the same plane, where, for example, a CCD or film can be placed. Thus, such a system is verifocal, producing images of different magnifications.
In some embodiments, the optical switch is a stack of films, which transmit or reflect by interference, controlled by an electric field. Additionally, one or more of the films can be backed by a polished optical flat, in order to flatten the film surface, to enhance image quality.
In some embodiments, the optical switch is a dichroic beamsplitter, sending light of some colors through only the first set of optics and light of other colors through both sets. In some embodiments, a color imager, such as a three-color CCD, is used to demultiplex the two, different, images.
Thus, an optical system can produce images (or beams, in the case of an afocal system) of different magnifications with no moving parts, either switching between two images or producing two images in two different color bands.
Other objects, advantages, and novel features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.